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Re: repairing a remote voltage monitor circuit.

Subject: Re: repairing a remote voltage monitor circuit.
From: Franc Zabkar
Date: Sun, 23 Oct 2005 08:28:27 +1000
Newsgroups: sci.electronics.basics, sci.electronics.repair
On Sat, 22 Oct 2005 04:23:25 GMT, mroberds@xxxxxxxxxxxxxxxx put finger
to keyboard and composed:

>In sci.electronics.repair Andy Baxter <news4@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> 
>wrote:
>>I've been helping out a local environmental centre by trying to fix a
>>remote voltage monitor circuit they have with their small windmill /
>>battery bank.  [...]  Does anyone have any general advice on how to
>>approach something like this?
>
>Here's what I'd try.  First, the meters can be loop powered (two wires)
>or locally powered (four wires) - look at the meter connections.  If
>they're supposed to be locally powered, make sure that power supply is
>working.  It also wouldn't surprise me if this setup brings +24 V from
>the top of the hill to power the meters separately from the signaling
>loop; if the +24 V doesn't make it down there, the meters won't work.
>
>If that checks out, then: Assuming it's 4 to 20 mA signaling, get a 24 V
>power supply or 24 V worth of (small) batteries.  Connect a 1 K resistor
>and 5 K pot in series, and connect this across the 24 V supply.  You can
>now draw 4 to 24 mA (at 24.0 V nominal) from the supply.  (If you don't
>like going over 20 mA, use a 1.2 K resistor.)  Go to where the meters are,
>disconnect one of the meters from the signal wires that go up the hill,
>and put the meter in series with the pot, resistor, and batteries.  The
>meter will have some internal resistance, so you won't be able to go to
>the full 24 mA, but you should be able to vary the reading on the meter
>by adjusting the pot.  If you can't make the meter respond, then there
>is a fault in the meter.
>
>If you can make all of the meters move with your test rig connected
>directly to the meters, try the same test, but from the top of the hill.
>Disconnect the signal cable from the electronics up there and use your
>battery-resistor-pot rig to send a varying current down the cable.

I'd leave the rig at the bottom of the hill. At the top of the hill
I'd put a 100 ohm resistor between the two ends of the signal cable.
That would enable me to detect shorts in the cable, and to watch the
meter while I'm adjusting the loop current.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

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